'If you put a frog in a pan of cold water and raise the temperature ever so slowly, it will doze happily ... and eventually cook to death.' Photograph: Corbis

You must know about penguins falling over onto their backs while trying to watch planes fly over, and that you can boil a frog without it trying to escape if you do it gently. You've probably heard that microwaving food in plastic containers can release cancer-causing agents, while a tooth left in a glass of Coca-Cola will dissolve overnight. And, of course, Nasa is famous for spending vast sums developing a pen that would write in space while the Russians simply used pencils.

The original Coca-Cola
did contain a tiny amount of cocaine, but a tooth left in a glass of coke overnight will not dissolve. Photograph: Frank Baron
All these stories have at least two things in common. The first is that they are not true. They're "urban legends" endlessly repeated and often embellished over the years. The second is that they are all
debunked on the Snopes website run by a husband-and-wife team, Barbara and David Mikkelson. The couple found one another via the alt.folklore.urban newsgroup, where this kind of thing has been discussed for years.

Not all the stories on Snopes are myths. It's true, for example, that in the early days Coca-Cola contained a tiny amount of cocaine, and that "a freighter carrying tapioca nearly sank when a fire in its hold (and the water used to extinguish it) cooked the cargo."

It's also true that sex expert Dr Ruth trained as an Israeli sniper, though she never killed anyone. "I was incredibly accurate throwing hand grenades too. Even today I can load a Sten automatic rifle in a single minute, blindfolded," she adds.

Stories are coded using traffic-light colours: red for false, amber for undetermined, and green for true. Some have "multiple truth values" while a great many get a grey spot for "unclassifiable veracity".

The site specialises in the kind of stuff circulated in emails, so Snopes is the place to go if you want to look up whether a claim is true or a virus warning is a hoax (shortcut: it's a hoax). And, of course, if you haven't already received email lists of "humorous automobile accident insurance claims", silly answers to chemistry tests and amusing newspaper headlines, Snopes has collected them for you.

Although it's mostly text, there is a "Fauxtography" section for images and videos. Again, they're rated for truthiness, and there are some fascinating stories. One video, for example, shows a TV reporter being run over. It turns out to be a shortened version of a KwikFit tyre commercial featuring "Dutch weatherman Piet Paulusma, who is quite alive and uninjured."

It can take a long time to dig up and reference the facts behind the stuff in emails and "virals", but Barbara Mikkelson is doing it for us.

The Snopes site is straightforward, being a list of categories with subdivisions. The categories range from Autos to Weddings via Food, Love, Religion and so on. If you fancy a browse, this is a quick way in. But if you want to look something up, it's usually quicker to use the Search page.

This is worth doing, because checking the veracity of emails makes you look smart, whereas blindly forwarding them to all your friends can make you look gullible, or worse.